John Campbell, a physicist with an interest in light and
colour in the atmosphere, responded.

A rainbow is the coloured bow we see whenever raindrops are
illuminated by the sun behind us.

When a ray of white light from the sun enters a spherical
droplet of water it travels through the droplet, reflects
off the rear surface and re-emerges from the front surface.
If we look away from the sun we see a circular bright bow
that appears to be at the same angle across as that formed by
about four hand spans at arm's length.

The colour display of the bow comes about because in water blue light
travels slower than red light so a blue ray is bent through a larger angle
than the red ray. Hence the outside of the bow appears iridescent red with
the other colours predominating in turn through to violet on the inside of
the bow.

The angle subtended by the rainbow is specific to water because that is
what all raindrops are made of. You can produce your own rainbow in a
darkened room by shining a torch at a clear plastic bottle containing
water. Look for the colour display at the edge of the bright region reflected
back at an angle of 42 degrees from the torch beam onto, say, a white wall
or a piece of white paper. Measure this angle, or the distance between the
coloured displays on opposite sides of the torch beam. Then replace the
water bottle with similar smooth walled plastic bottles of cooking oil,
kerosene etc. They too produce rainbows but of smaller size because light
travels slower in these fluids than it does in water.